The Microlite 500 would IMHO only be suitable for indoor use,
I had a look at one yesterday at their Plymouth store and it would be okay for summer use in the UK in a tent or bivibag. I’ve slept out in the rain in July in an even less insulated Trekmates Microfleece sleeping bag liner and a bivi bag and was more than comfortable (the round-the-campfire scrumpy session might have helped though ;-) )
You can often pick up the liners at T K Maxx for under a tenner.
Trekmates Microfleece
I’m going to test my Snugpak Travel Lite bag (along with the Trekmates liner) up on Dartmoor next weekend to see how that’ll cope with the cooler Spring temperatures. The Snugpak bag’s intended for warmer foreign travel and UK Summer use, but it packs down tiny. I’ll be taking emergency thermal underwear as backup though :-)
Snugpak Travel Lite
BTW, Mountain Warehouse have LS Merino zip collar baselayers reduced to £20 at the moment. Like all MW clothes the styling and cut looks a bit suspect but they might do if you’re currently after something like that.